Q.I’ve heard that
engineers give no structural "credit" to gypboard, but
I know it greatly stiffens partitions when I nail it
up. How much shear strength does drywall really have,
and why not credit it in the design?

A. As you suspect, properly
fastened gypboard does have the capacity to resist
racking and/or lateral forces. The 1997 Uniform
Building Code (Table 25-1) gives shear values for
both gypsum wallboard and gypsum sheathing. In
fact, the allowable lateral force on a wall with
fully blocked 5/8-inch gypboard on both sides
nailed at 4-inch centers (350 plf) actually exceeds
that of a wall with 1/2-inch Structural I plywood
fastened with 10-penny nails at 6-inch centers (340
plf). Be careful, though: If you are working in
seismic Zones 3 or 4, note that even with fully
blocked edges you must reduce the allowable lateral
load on gypboard by 50%.

As to crediting the design for the strength of
the gypboard, this decision is based on the
materials selected for the particular structure. If
you build a house with rigid-foam insulation panels
on the exterior (under finishes) and gypboard on
the interior, then the gypboard is the
lateral force-resisting material. However, if the
interior gypboard is combined with plywood
sheathing on the exterior (or with diagonally
braced structural steel studs), then the strength
of the gypboard is discounted. In the latter case,
the plywood is considered the primary
lateral-force-resisting material because of its
greater strength and stiffness. In both instances,
the designer must make certain that the primary
lateral-force-resisting material is sufficiently
fastened to the framing to resist the
total lateral load despite the presence of
other secondary materials.

In reality, it is the combination of all the
primary and secondary materials that will resist
the applied lateral loads. However, should the
loading persist, the repetitive cycles of
load/release will cause fatigue of the weaker
materials (like gypboard) until essentially only
the primary lateral material remains functional. If
we were to credit the strength of the gypboard
towards the total lateral load (and reduce the
plywood nailing accordingly), our structure would
lack critical capacity after the time when the
gypboard had yielded. This is the reason gypboard
receives no credit for its strength.